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Submerged early Holocene coastal and terrestrial landforms on the inner shelves of Atlantic Canada
Quaternary International 206 (2009) 24–34
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Submerged early Holocene coastal and terrestrial landforms on the inner shelves of Atlantic Canada John Shaw a, *, Gordon B. Fader b, Robert B. Taylor a a b
Natural Resources Canada, Geological Survey of Canada, Atlantic, Bedford Institute of Oceanography, 1 Challenger Drive, Dartmouth, Nova Scotia, Canada B2Y 4A2 Atlantic Marine Geological Consulting Ltd., Halifax, Nova Scotia, Canada B3L 2Z2
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 22 August 2008
Coastal and terrestrial landforms that formed with lowered relative sea levels during the early postglacial period in Atlantic Canada were submerged during the Holocene transgression. However, these landforms are seldom seen on sea floor imagery. Factors that contribute to their destruction include the brevity of sea level lowstands and high wave energy on shallow modern shelves. We identify one situation within which preservation has been relatively good: large coastal lakes that existed for many thousands of years before being connected to the ocean by rising sea level in the mid-Holocene. We describe Bedford Basin, near Halifax, Nova Scotia, and deal more exhaustively with the Bras d’Or Lakes, an inland sea in Cape Breton, Nova Scotia. The preservation of shore platforms, barrier beaches and spits, and fluvial systems, was due to the rapid onset of the transgression and the relatively low wave energy in the subsequent marine phases. The well-preserved early Holocene coastlines are highly favourable targets in the search for evidence of human occupation in the early- to mid-Holocene. Ó 2008 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Although large areas of the continental shelves around Atlantic Canada (Fig. 1) were emergent at various times in the postglacial period (Shaw et al., 2002a), geophysical surveys rarely yield evidence of submerged shorelines. This is arguably due to the effects of high wave energy during the Holocene transgression. On the high wave energy Atlantic coasts of Nova Scotia, for example, where relative sea level (rsl) is rising at about 36 cm/century (Shaw et al., 1993), extra-tropical storms and hurricanes trigger rapid migration of coastal systems and erosion of coastal bluffs (e.g. Boyd et al., 1987; Carter et al., 1987, 1990, 1992, 1995; Forbes et al., 1990; Orford et al., 1991). The ongoing transgression destroys coastal landforms, leaving behind only truncated and buried estuarine deposits below thin surface lags (Forbes et al., 1991), or drumlin scars (Wang and Piper, 1982). Shaw (2005) described evidence of fluvial, deltaic and coastal systems on the shelves of Atlantic Canada. He argued that although large areas of the outer continental shelves were emergent for many thousands of years (see maps in Shaw et al., 2002a), lowstands were of insufficient duration to develop large coastal landforms that might survive transgression. Nevertheless, coastal landforms had been preserved in special circumstances (Forbes et al., 1995), and remnant early Holocene
* Corresponding author. Tel.: þ1 902 4266204; fax: þ1 902 4264104. E-mail address: [email protected] (J. Shaw). 1040-6182/$ – see front matter Ó 2008 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2008.07.017
fluvial systems have been found in a few areas, notably between New Brunswick and Prince Edward Island (Shaw, 2005). Submerged early Holocene deltas are found in southern Newfoundland, where they were created and preserved by unique circumstances, namely rapid relative sea level fall to a brief lowstand, abundant sediment supply in fiord-head glaciomarine deltas, and low wave energy (Shaw and Forbes, 1995). The settings in which well-preserved early Holocene deltas, fluvial channels, and coastal landforms are most likely to be found are large marine embayments that were formerly coastal freshwater lakes in the early Holocene (Shaw, 2005). In this paper we describe coastal and other landforms that have been preserved in two such settings: Bedford Basin, near Halifax, Nova Scotia, and the Bras d’Or Lakes, an inland sea in Cape Breton, Nova Scotia (Fig. 1). We examine the factors that led to the creation and preservation of these early Holocene shorelines and coastal landscapes, and note the implications for archaeology. 2. Study areas In this paper we focus on two areas: Bedford Basin and the Bras d’Or Lakes, both in Nova Scotia, Canada (Fig. 1). Bedford Basin (Fig. 2) is a large, semi-enclosed bay measuring approximately 7 km by 3 km, with a maximum depth of 71 m in the centre of the bay. Tidal range is up to a maximum of 2 m, and wind waves in the basin seldom exceed 1 m in height due to the restricted fetch. The basin is connected to Halifax Harbour and the Atlantic Ocean by The
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Fig. 1. Map showing location of Bedford Basin (A) and the Bras d’Or Lakes (B).
Narrows, a constricted 20-m deep channel, but was a freshwater lake until the mid-Holocene (Miller et al., 1982). The marine geology of the basin was described by Fader et al. (1991), Fader and Buckley (1995), Miller and Fader (1995) and in a forthcoming Geological Survey of Canada (GSC) Bulletin (Fader and Miller, in press). The Bras d’Or Lakes is the unofficial name for the inland sea on Cape Breton Island, Nova Scotia (Fig. 5) that comprises two large bodies of water: Bras d’Or Lake, in the south, and Great Bras d’Or, in the north. They are connected by Barra Strait. The principal connection with the ocean, Great Bras d’Or Channel, provides 95% of the exchange of marine water and has a sill depth of about 8 m (Canadian Hydrographic Service, 1996). The lakes are saline, but salinity decreases to about 21 parts per thousand in places (Petrie and Bugden, 2002). The circulation is predominantly inwards at the sea floor, and seawards at the surface. Tidal range is attenuated to only a few centimeters. In the last several years, the lakes have been surveyed using multibeam bathymetry systems, with the goal of producing 1:50,000 scale maps of bathymetry, backscatter and surficial geology. In addition to a suite of glacial landforms, the new imagery revealed extensive evidence of submerged shorelines, wave-cut platforms, river valleys and deltas (Shaw et al., 2006). The preserved features are mostly 25 m below modern sea level, far below the modern sill at 8 m. 3. Methods Bedford Basin (Figs. 2–4) was surveyed by the Canadian Hydrographic Service using multibeam bathymetry systems. A 5-m gridded data set supplied by the Department of Fisheries and Oceans Canada was viewed and analyzed in the Global Mapper Geographic Information System (GIS). Bedford Basin has been surveyed with various acoustic systems and a comprehensive suite of bottom samples has been obtained (Fader and Miller, in press). Multibeam bathymetric data were collected in the Bras d’Or Lakes (Fig. 1) in 2002, 2003 and 2004 using Canadian Hydrographic
Service launches with a Simrad EM-3002 system, and the survey vessel CCGS Creed, deploying a Simrad EM-1002 system. Denys Basin (Fig. 5) was surveyed in 2004 by a hydrographic survey launch with a SEA interferometric sidescan sonar system. Data were cleaned using Caris HIPS (Hydrographic Information Processing System). Processed depths were imported into the Geological Survey of Canada (GSC) version of the US Army Corps of Engineers GRASS GIS system. They were gridded, given sunillumination, and analyzed. Multibeam and interferometric data were processed to extract backscatter, a proxy for sediment type. Ground-truthing information came from a geophysical survey by CCGS Dawson in 1985 (Lynch, 1995), a geophysical survey by the first author in 1995 using CCGS Hart, gravity coring and geophysical surveys in the fall of 2002 (Parrott, in press), and a geophysical and sampling survey in 2003 (Shaw et al., 2005). One core was analyzed for foraminifera, and several were analyzed for macrofossil content by Paleotec Services of Ottawa, Ontario. Radiocarbon dates were calibrated with Calib 5.0.1 (Table 1). GSC Open File 5397 (Shaw et al., 2006) contains an ArcGIS project with images of the multibeam bathymetry and backscatter for the Bras d’Or Lakes, and an image with water level at 25 m. It also contains a detailed report on methods and results, a detailed description of coastal types, and an analysis of future water level trends and impacts. An understanding of modern coastal morphology and dynamics was based in part on aerial video surveys (Taylor and Frobel, 1998) and ground surveys (Taylor and Frobel, 2005). 4. Environmental change in Bedford Basin 4.1. Water levels In the early Holocene, relative sea level was low in the Halifax area (see Shaw et al., 1993; Edgecombe et al., 1999) and the basin existed as a lake. Micropaleontological data show that because of rising sea levels the lake became marine (Miller et al., 1982; Fader and Buckley, 1995, 1997). The transition to marine sedimentation is
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Fig. 2. Multibeam image of Bedford Basin. The inset map shows the extent of the freshwater lake during the early Holocene. To the northwest of the white dashed line the early- to mid-Holocene shoreline is buried by mud. The white arrow indicates the location of a bedrock ridge that was exposed during the relative sea level lowstand. The large white box delineates the extent of Fig. 3 and the small white box that of Fig. 4. Data courtesy of the Canadian Hydrographic Service.
dated at 5830 230 14C yr BP (Miller et al., 1982). Based on the sill depth at The Narrows, it was estimated that relative sea level had risen w20 m since that time. Part of the Halifax relative sea level curve is shown in Fig. 7. 4.2. Changes in geography The margins of Bedford Basin (Fig. 2) are somewhat rugged, with sea floor exposures of bedrock and thin till (Fader and Miller, in press). The centre of the basin is deep and muddy, with low-relief. A gently sloping terrace eroded into till, with an inflexion at 21 m below Chart Datum (CD) can be traced around the basin several hundred metres offshore; it can be traced around several former islands on the west side of the basin (Fig. 3). The upper limit of the terrace is a 0.5 to 1.0 m high boulder berm, while its lower limit is a line of boulders with little or no relief (Fig. 4). We believe that the terrace formed as a result of wave-erosion, and is indicative of an early Holocene lake level at 21 m CD. The extent of the former lake is shown on the inset in Fig. 2. Chart datum is approximately the level of modern low tides, so the former lake level was w22 m below modern mean sea level. The upper berm was probably created by a combination of ice-push and thermal expansion (see Gilbert, 1990) in a lake setting over an
extended period. Several hypotheses were considered for the lower line of boulders, namely: (1) it is a boulder barricade (Rosen, 1979) that rapidly formed (Gilbert, 1990) when the lakes became tidal; and (2) it formed when lake level dropped slightly as the sill was being breached by rising sea levels. The most likely explanation is that this low-relief boulder ridge is the exposed edge of a bouldermantled till surface that is buried by postglacial mud at a level deeper than the terrace (Fig. 4). The submerged shoreline disappears below a cover of lateHolocene mud in the northwest of the basin, in Bedford Bay near the channel of the Sackville River (i.e. beyond the dashed line on Fig. 2). A series of bedrock ridges extending across the basin in this region (arrow on Fig. 2; see also inset map) impounded a second lake during the early Holocene. It stood a few metres higher than the main lake and drained into the latter via a narrow rocky channel with rapids or waterfalls. 5. Environmental change in the Bras d’Or Lakes 5.1. Water levels Analyzing data from the 1985 survey (Lynch, 1995), HillaireMarcel (1987) reported that during the postglacial period the lake
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Fig. 3. Submerged shorelines on the west side of Bedford Basin. Data courtesy of the Canadian Hydrographic Service.
had experienced a marine phase, a freshwater phase, and a final marine phase (see also Lortie, 1987). The onset of the final freshwater phase was undated. De Vernal and Jette´ (1987) concluded that the onset of freshwater conditions was ca. 4500 to 5000 14c yr BP. Given that the sill at the mouth of the lakes was at a depth of 8 m, it was estimated that the rate of relative sea level increase since the onset of marine conditions had been 16–20 cm/ century. Shaw et al. (2002b) tried to reconcile Grant’s (1994) conclusions that late glacial sea levels were low in the area with Hillaire-Marcel’s (1987) evidence that they were high at the beginning of the Holocene. They argued that the early Holocene freshwater lake in
the area stood 25 m below modern sea level. Because the modern sill in Great Bras d’Or Channel is only 8 m deep they proposed that the freshwater lake drained to the ocean via a minor Channel, Little Bras d’Or Channel (Fig. 5). Although the modern sill connecting the Bras d’Or Lakes (Fig. 5) with the Atlantic Ocean, in Great Bras d’Or Channel, is only 8 m deep, sub-bottom profiling in the channel reveals that a former bedrock sill at 25 m is buried beneath a prograded wedge of sand (Fig. 6, top) interpreted as an elongated flood-tidal delta (Shaw et al., 2006). The source of sediment in this delta is sand eroded from high, unconsolidated bluffs immediately outside the entrance to the lakes (Taylor and Frobel, 2005) and transported into the channel by tidal currents that have a strong
Fig. 4. Sidescan sonogram showing the submerged shoreline, comprising two lines of boulders and intervening terrace. Light tones are less reflective sea floor (mud) while dark tones are highly reflective (boulders, gravel).
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Fig. 5. The Bras d’Or Lakes, with locations of figures, and locations of the two seismic profiles shown in Fig. 6. Inset map shows the extent of freshwater lakes in the early Holocene.
landward asymmetry at the seabed (Petrie and Bugden, 2002). This evidence shows that the former sill to the lakes is 25 m below modern sea level, and not 8 m as might be supposed. Coring in St. Patrick’s Channel (Fig. 5) reveals marine mud up to 3 m thick containing scattered shell fragments, marine bivalves (Cerastoderma pinnulata), and estuarine foraminifera whose abundance decreases downcore. The marine mud was deposited under conditions of increasing marine influence through time. Underlying the mud are layers of gray clay interbedded with peat and gyttja containing twigs and tree leaves, with seeds of aquatic plants such as Typha sp., Sparganium americanum and the shrub Alnus incana. A few foraminifera were noted in some parts of the organic-rich units, but not in others. These organic-rich units accumulated in fresh and brackish conditions, probably in a series of marshes and ponds on the flat floor of what is now St. Patrick’s Channel. The generalized relative sea level curve for the lakes (Fig. 7) is primarily based on nine samples from the organic layers (Shaw et al., 2006), together with two dates on shells in the overlying
marine mud (Table 1). Four dates (12–15) published by Miller and Livingstone (1993) have been added. These dates come from a small cove mid-way along Great Bras d’Or Channel, connected to the channel via a sill that is 1.5 m below the high tide level. The dates showed that sea level was 1.5 m below present level at 950 BP. 5.2. Geographic changes The inset on Fig. 5 shows the geography of the Bras d’Or Lakes region during the early- to mid-Holocene, based on multibeam bathymetry data with a 25 m lowering of water level applied. In the north, St. Patrick’s Channel was subaerially exposed, and contained a river that drained into the lake at the east end of the channel, where it formed a large delta (Fig. 6, bottom). At the west end of the channel, modern basins were occupied by lakes at higher levels. In the southern lakes, modern East Bay contains several basins that, in the early- to mid-Holocene, contained lakes at elevations slightly above elevations of the main lake (24 m), and were
Table 1 Radiocarbon dates on samples from St. Patrick’s Channel, Bras d’Or Lakes. #
Core and depth
Material dated
Depth (m)
Lab #
13
Age
Error
Calibrated age BP (two sigma range)
1 2 3 4 5 6
2002066-090-070 2002066-090-140 2002066-090-152 2002066-090-160 2002066-094-123 2002066-094-155
Shell Twig Wood Twig Peat Wood
14.3 15.0 15.1 15.2 13.2 13.6
185984 197660 185985 197661 185986 185987
C/12C ratio
0.3 25.3 28.1 26 27.7 27.3
2120 4440 4510 4480 4110 4090
40 40 40 40 40 40
7 8 9 10 11
2002066-096-074 2002066-096-122 2002066-098-064 2003015-077-160 2003015-077-170
Shell Peat Peat Wood Twig
13.9 14.4 11.5 21.8 21.9
185988 185989 185990 185991 197662
0.5 28.1 27.7 28.2 28.2
2330 4220 3900 5250 5370
60 40 40 40 40
1411–1659 4876–5083; 5039–5310 4975–5017; 4453–4461; 4442–4483; 4718–4725; 1621–1931 4624–4763; 4160–4168; 5922–6121; 6004–6083;
5103–5138; 5162–5282 5031–5297 4520–4729; 4735–4741; 4750–4821 4512–4711 4753–4815 4787–4857 4179–4199; 4229–4428 6146–6178 6099–6160; 6170–6280
Numbers in column one refer to locations illustrated in Shaw et al. (2006). Column two shows the core number and sample depth down core. Sample depths are reduced to Chart Datum, which is lowest normal tide. Tidal range is about 0.1 m. Calibrated ages were obtained using Calib 5.0.1. The woody samples were calibrated using intcal04.14c and the shell samples using marine04.14c with local Delta R ¼ 140 (McNeely et al., 2006).
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Fig. 6. Acoustic profiles in the Bras d’Or Lakes. Top: Seistec high-resolution sub-bottom profile across the flood-tidal delta inside the entrance to the lakes. (A) Flood-tidal delta; (B) bedrock; (C) masking by shallow gas. Bottom: Huntec high-resolution deeptowed sub-bottom profile across the delta at the mouth of St. Patrick’s Channel. (D) Postglacial marine mud; (E) glaciomarine and glaciolacustrine/lacustrine mud; the upper part of this sediment package grades into the delta foresets (F) at the west side of the profile; (G) bedrock or possibly till.
connected to it by rivers. Denys Basin (Fig. 5) was emergent, and was drained by a meandering river that formed a large delta where it reached the lake. Large fields of drumlins in the modern West Bay formed a complex network of islands connected by barrier beaches, with narrow, winding channels that connected with the main part of the ancient lakes. 5.3. Submerged coastal landforms The Bras d’Or Lakes existed for many thousands of years as freshwater lakes, and developed coastal landforms easily identifiable on multibeam sonar imagery and similar in size, morphology and lithology to their modern counterparts in the lakes (see Woodman, 1899; Taylor and Shaw, 2002; Taylor and Frobel, 2005). The greater abundance of coastal landforms in the southern lake (Bras d’Or lake) is explained by the prevalence of drumlin fields
Fig. 7. Simplified trend of relative sea level in the Bras d’Or Lakes during the middle- to late-Holocene epoch. The curve is poorly constrained after ca. 4000 BP. Samples 12–15 are from Miller and Livingstone (1993), who argued that sea level was 1.5 m below present at 950 BP. The curve for Halifax (dotted line) is based on Edgecombe et al. (1999).
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there, and their absence in the northern lake (Great Bras d’Or). These are the underwater extension of drumlin fields mapped on land by Grant (1988, 1994) and Stea et al. (1992), which acted as sediment supplies for coastal landforms (Taylor and Shaw, 2002; Shaw et al., 2006). East Bay (Fig. 8) contains tombolos, spits, and barrier beaches, mostly graded to the level of 24 m, slightly shallower than the lakes north of Barra Strait (25 m). They have high backscatter, and are composed of sub-rounded pebbly gravel. They appear on seismic profiles superimposed on earlier deposits. In some instances barrier beaches are located at shallower depths, indicating that with rising sea levels they migrated until they were trapped by topography (see Fig. 8D). In West Bay, fields of drumlins were modified by littoral processes to produce coastal landforms that were subsequently submerged. They include barrier beaches linking drumlin headlands (Fig. 9). They have high backscatter, and formed with a 24 m water level, again slightly shallower than north of Barra Strait. West Bay also contains extensive submerged shore platforms at 24 m. These are incised into drumlins, and formed under the influence of waves generated by a relatively large (12 km) fetch to the southwest. The platforms are poorly developed on the northeast sides of drumlins. Except for possibly one area, ridges at the upper limits of the platforms (as in Bedford Basin) are absent. A distinctive aspect of West Bay is the presence of ‘tails’ attached to submerged drumlin shoals (Fig. 9). During the lake phase some drumlins had trailing spits attached. As sea level rose, the spits migrated to the rear of the shoals, coalesced, and formed attached ‘tails’ that were eventually trapped by topography and drowned. The tails are generally elongate, but in places the sediment trapped in the lee of a drumlin forms a spillover wedge. In a conceptual model for barrier inception, growth and destruction in a rising sea level setting in the Bras d’Or Lakes (Taylor and Shaw, 2002; Shaw et al., 2006), these ‘tails’ represent the stranding of sediment in the lee of a drumlin that has become completely eroded and submerged. Modern examples are seen in Fig. 59 of Shaw et al. (2006). 5.4. Submerged fluvial systems and deltas Well-preserved submerged river systems are found in the lakes. In St. Patrick’s Channel an incised river valley is bounded by bluffs and has a braided channel floor (Fig. 10). The former river formed a delta a short distance to the southeast. The thickness of the deltaic wedge (Fig. 6) and its interfingering with strongly stratified sediments interpreted as both glaciomarine and glaciolacustrine in origin, suggests that it was initially sourced by glacial meltwater, when sea level had dropped below the 25 m sill level. Farther west the channel is overlain by mud, but can still be glimpsed on multibeam sonar imagery as a series of looped meanders. Yet farther west the channel is imaged below the 3-m thick mud on sub-bottom profiles. The low-gradient valley floor was poorly drained, and hosted marshes and ponds in which organic materials accumulated – i.e. the materials sampled and dated for the sea level curve. A second submerged fluvial channel system is seen in Denys Basin (Fig. 11). Knudsen 28 kHz sounder data show that the channels lie below a thin blanket of late-Holocene mud. This river reached the former lake and formed a delta, which today is an extensive flat area at a depth of 24 to 25 m. The surface has a mud veneer, through which meandering channels are vaguely discernable. The size of this delta suggests that it also was partly formed by meltwater discharged by glacial ice located farther west. 5.5. Submerged bioherms Surveys in Denys Basin using the interferometric sidescan system revealed numerous acoustically reflective, circular to oval
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Fig. 8. Multibeam bathymetry image showing submerged coastal landforms in East Bay. These include a tombola (A), spits (B), barrier beaches (C), and a relatively shallow barrier beach (D) that was stranded during the process of migration. The structures at E are probably barriers enclosing lagoons, and are commensurate with the modern analogs in the region (see Taylor and Shaw, 2002). All these landforms have high backscatter, and bottom photographs and samples show that they are composed of gravel or sandy gravel.
Fig. 9. Multibeam image from West Bay, showing the submerged erosional terraces (A) together with a barrier beach (B). When the lake stood at 24 m, the narrow channel at C connected the lake in the southwest (modern West Bay) with the main lake. The trailing ‘tails’ attached to submerged drumlins are shown at D.
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Fig. 10. Multibeam image of a submerged river system in St. Patrick’s Channel. The channel terminates in a delta at 25 m just off the image at right. It appears to show gravel bars (A) bounded by bluffs (B). Towards the southwest the channels disappear beneath a cover of mud, but can be vaguely discerned on the multibeam imagery as a series of looped meanders, and an oxbow (C).
marks on the sea floor (Fig. 12, top). Bottom video and sampling (G. Bugden, personal communication, 2006) reveals that these features are circular to oval bioherms of the American oyster, Crassostrea virginica. These contrast with the linear bioherms reported in New England by Carbotte et al. (2004). Coring reveals the bioherms as vertical structures composed of oyster shells and interstitial mud, surrounded by mud. Sub-bottom profiles with a Knudsen dualfrequency sounder (Fig. 12, bottom) show that the bioherms are commonly 3 m high, and are ‘rooted’ on a hard substrate below the acoustically transparent Holocene mud. 6. Discussion 6.1. Erosional terraces There are significant differences between the submerged erosional terraces in the two study areas. The Bras d’Or Lakes submerged terraces are relatively wide (80 m) and formed in an environment with large fetch (up to 12 km) and abundant sediment supply in the form of numerous drumlins. As a result, coastal barriers and spits were as common in the lake as they are in the modern inland sea. The abundance of sediment is reflected in the presence of the drumlin tails and spillover deposits that were trapped as individual drumlins were submerged. The Bedford Basin shoreline, by contrast, was developed in an area of limited fetch (2 km), and thin till, so coastal landforms did not develop. The boulder ridge at the upper edge of the Bedford Basin terraces may represent a back-terrace ridge that was abandoned during the rapid transgression.
6.2. Drowning and preservation of coastal landforms The drowning of early Holocene shorelines at the two study sites is the result of the instantaneous onset of rapid sea level rise, when the Holocene transgression breached the sills. When the sills at Bedford Basin and the Bras d’Or Lakes were breached (ca. 6650 and 6350 yr cal BP), the respective rates of sea level rise were about 67 and 79 cm/century, respectively. For comparison, the modern rate of sea level rise at Halifax, Nova Scotia, is about 36 cm/century (Shaw et al., 1993). Both water bodies were thus subject to an instantaneous and rapid increase in water level, accounting for the good preservation of coastal landforms. Although most coastal landforms in the Bras d’Or Lakes drowned rapidly, there is, nevertheless, evidence that some adjusted to sea level rise by migrating. Coastal features identifiable on the multibeam imagery are at depths from 7 to 25 m. The majority are in the range 15–20 m, with a nearly equal number in the range 20–25 m. Drowning of coastal systems thus occurred mainly before 4500 cal yr BP, and most drowning was in the period 5500 to 6000 cal yr BP. The rate of water level increase at 4500 BP was 57 cm/century, and from 5500 to 6000 BP it ranged from 69 to 75 cm/century. 6.3. Oyster bioherms The oyster colonies clearly formed after 6350 yr cal BP, when the lakes became connected with the ocean. The river channels in this part of Denys Basin extend to depths of 14 m, and would have become brackish ca. 4500 cal yr BP. The bases of the bioherms are on average 8 m below sea level, suggesting that they could have
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Fig. 11. Multibeam image showing the submerged fluvial system in Denys Basin, buried under several metres of postglacial mud. Fig. 12 is located just off this figure, in the extreme southwest.
Fig. 12. Top: backscatter in part of Denys Basin, derived from an interferometric survey system. The black circles and ovals are oyster bioherms – colonies up to 25 m in diameter. Bottom: sub-bottom reflection profile across the bioherms. The horizontal dashed lines are 2 m apart.
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been established after 3500 cal yr BP. We infer, therefore, that oysters could have migrated into this area after 4500 cal yr BP, and were possibly established by 3500 cal yr BP. 6.4. Archaeological implications Murphy (1998) noted that in the prehistoric culture sequence of the Maritime Provinces of Atlantic Canada (New Brunswick, Nova Scotia, and Prince Edward Island) there are no professionally excavated sites dating to the Early and Middle Archaic periods (i.e. 10,000–5000 BP). It is likely that occupational evidence was formerly located along shorelines that were submerged during the Holocene transgression. As noted in Section 1, the high wave energy of the transgression destroyed former coastlines. However, the two examples described here show that ancient coastlines have been well-preserved in the special circumstances of large coastal lakes that were stable for many thousands of years, until they connected with the ocean in the mid-Holocene. Based on the concepts elucidated by Pedersen et al. (1997) it should be possible to identify locations with a high potential for former human occupation in both the Bras d’Or lakes and Bedford Basin. 7. Conclusions (1) In the early Holocene, Bras d’Or Lakes and Bedford Basin, Nova Scotia were freshwater lakes. They connected with the ocean at ca. 6350 and 6650 cal yr BP, respectively, due to rising sea levels. (2) Shoreline systems that developed in these large lakes over many thousands of years were preserved due to instantaneous onset of rapidly rising water levels. This contrasts with the more exposed continental shelves in the region, where wellpreserved coastal systems are rare. (3) The submerged shorelines in Bedford Basin consist of shore platforms at 21 m; more complex shores at around 25 m and above were developed in the Bras d’Or Lakes because of the presence of drumlin fields that acted as sediment sources. Some coastal systems were able to migrate landward for a while before drowning. (4) We believe that these submerged landscapes and features provide high-priority targets for the search for early human occupation of the region.
Acknowledgements We thank David Frobel for assistance with coastal videos and coastal fieldwork. Russell Parrot and David Piper provided helpful internal reviews. Stanley Johnson of the Department of Fisheries and Oceans Canada (DFO) provided data files for Bedford Basin and Gary Bugden (DFO) gave information on the bioherms in Denys Basin. This research was part of Natural Resources Canada Project X29 in the Geoscience for Ocean management Program and is Earth Sciences Sector Contribution 20070549. References Boyd, R., Bowen, A.J., Hall, R.K., 1987. An evolutionary model for transgressive sedimentation on the Eastern Shore of Nova Scotia. In: Fitzgerald, D.M., Rosen, P.S. (Eds.), Glaciated Coasts. Academic Press, San Diego, pp. 87–114. Canadian Hydrographic Service, 1996. Chart 4277: Great Bras d’Or, St. Andrews Channel, and/et St. Anns Bay, scale 1:40,000. Carbotte, S.M., Bell, R.E., Ryan, W.B.F., McHugh, C., Slagle, A., Nitsche, F., Rubenstone, J., 2004. Environmental change and oyster colonization within the Hudson River estuary linked to Holocene climate. Geo-Marine Letters 24, 212–224. Carter, R.W.G., Shaw, J., Jennings, S.C.,1995. Morphodynamic evolution, self-organisation and instability of coarse-clastic barriers on paraglacial coasts. Marine Geology 126, 63–85.
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Carter, R.W.G., Orford, J.D., Forbes, D.L., Taylor, R.B., 1987. Gravel barriers, headlands and lagoons: an evolutionary model. In: Kraus, N.C. (Ed.), Coastal Sediments ‘87: Proceedings of a Specialty Conference on Advances in Understanding of Coastal Sediment Processes: New Orleans, Louisiana, May 12–14, 1987. American Society of Civil Engineers, II, pp. 1776–1792. Carter, R.W.G., Orford, J.D., Forbes, D.L., Taylor, R.B., 1990. Morphosedimentary development of drumlin-flank barriers with rapidly rising sea level, Story Head, Nova Scotia. Sedimentary Geology 69, 117–138. Carter, R.W.G., Orford, J.D., Jennings, S.C., Shaw, J., Smith, J.P., 1992. Recent evolution of a paraglacial estuary under conditions of rapid sea-level rise: Chezzetcook Inlet, Nova Scotia. Proceedings of the Geological Association of London 103, 167–185. De Vernal, A., Jette´, H.,1987. Analyses palynologiques de se´diments holoce`nes du lac Bras d’Or (carotte 85-036-016P), ˆIle du Cap-Breton, Nouvelle-E´cosse. In: Recherches en cours, Partie A, Commission ge´ologique du Canada, E´tude 87-1A, pp. 11–15. Edgecombe, R.B., Scott, D.B., Fader, G.B.J., 1999. New data from Halifax Harbour: paleoenvironment and a new Holocene sea-level curve for the inner Scotian Shelf. Canadian Journal of Earth Sciences 36, 805–817. Fader, G.B.J., Miller, R.O. The surficial geology of Halifax Harbour. Geological Survey of Canada, Bulletin number 590, in press. Fader, G.B.J., Miller, R.O., Pecore, S.S., 1991. The marine geology of Halifax Harbour and adjacent areas. Geological Survey of Canada, Open File Report 2384, 2 vols., 23 p. and maps. Fader, G.B.J., Buckley, D.E., 1995. Environmental geology of Halifax Harbour, Nova Scotia. Geoscience Canada 22 (4), 152–171. Fader, G.B.J., Buckley, D.E., 1997. Environmental Geology of Halifax Harbour, Nova Scotia. In: Eyles, N. (Ed.), Environmental Geology of Urban Areas, 1997. Geological Association of Canada, GEOtext 3, St John’s, Newfoundland, pp. 249–267. Forbes, D.L., Boyd, R., Shaw, J., 1991. Late Quaternary sedimentation and sea level changes on the inner Scotian Shelf. Continental Shelf Research 11, 1155–1179. Forbes, D.L., Shaw, J., Taylor, R.B., 1995. Differential preservation of coastal structures on paraglacial shelves: Holocene deposits of southeastern Canada. Marine Geology 124, 187–201. Forbes, D.L., Taylor, R.B., Shaw, J., Carter, R.W.G., Orford, J.D., 1990. Development and stability of barrier beaches on the Atlantic coast of Nova Scotia. In: Proceedings, Canadian Coastal Conference (Kingston). National Research Council, Ottawa, pp. 83–98. Gilbert, R., 1990. A distinction between ice-pushed and ice-lifted landforms on lacustrine and marine coasts. Earth Surface Processes and Landforms 15, 15–24. Grant, D.R., 1988. Surficial Geology, Cape Breton Island, Nova Scotia. Geological Survey of Canada Map 1631A, scale 1:125,000. Grant, D.R., 1994. Quaternary Geology, Cape Breton Island, Nova Scotia. In: Geological Survey of Canada, vol. 482. Natural Resources Canada, Ottawa, 159 pp. Hillaire-Marcel, C., 1987. Composition isotopique du carbone organique des carottes du forage 85-036-016 dans le lac Bras d’Or, ˆIle du Cap-Breton, Nouvelle E´cosse. In: Recherches en cours, Partie A, Commission ge´ologique du Canada, E´tude 87-1A, pp. 889–896. Lortie, G., 1987. Diatome´es et evolution holoce`ne du lac Bras d’Or, Nouvelle E´cosse. In: Recherches en cours, Partie A, Commission ge´ologique du Canada, E´tude 87-1A, pp. 859–864. Lynch, D.W., 1995. Late Quaternary acoustic stratigraphy and glacial history of the Bras d’Or Lakes using seismic data and cores. Unpublished B.Sc. honours thesis, Geology Department, Saint Mary’s University, Halifax, Nova Scotia, 63 p. plus 2 maps. McNeely, R., Dyke, A.S., Southon, J.R., 2006. Canadian marine reservoir ages: preliminary data assessment. Geological Survey of Canada Open File 5049, CD. Miller, R.O., Fader, Gordon B.J., 1995. The Bottom of Halifax Harbour. Geological Survey of Canada Open File 3154 (poster). Miller, K.R., Livingstone, D.A., 1993. Late-Holocene changes in sea level and environment on eastern Cape Breton Island, Nova Scotia, Canada. The Holocene 3, 211–219. Miller, A.A.L., Mudie, P.J., Scott, D.B., 1982. Holocene history of Bedford Basin, Nova Scotia: foraminifera, dinoflagellate and pollen records. Canadian Journal of Earth Sciences 19, 2342–2367. Murphy, B.M., 1998. Researching the Early Holocene of the Maritime Provinces. Unpublished M.A. thesis, Department of Anthropology, Memorial University of Newfoundland, St. John’s, Newfoundland, 136 pp. Orford, J.D., Carter, R.W.G., Jennings, S.C., 1991. Coarse clastic barrier environments: evolution and implications for Quaternary sea-level interpretation. Quaternary International 9, 87–104. Parrott, D.R. Cruise Matthew 2002066, Geophysical surveys and sampling operations in the Middle Shoal Area, Cape Breton Island, Nova Scotia, 19– 30 October 2002. Geological Survey of Canada Open File Report 4992, in press. Pedersen, L., Fischer, A., Aaby, B., 1997. The Danish Storebælt Since the Ice Age – Man, Sea and Forest. A/S Storebælt Fixed Link, Copenhagen, 338 pp. Petrie, B., Bugden, G., 2002. The Physical Oceanography of the Bras d’Or Lakes. In: Petrie, B. (Ed.), Proceedings Nova Scotia Institute of Science, vol. 42, pp. 9–36. Rosen, P.S., 1979. Boulder barricades in central Labrador. Journal of Sedimentary Petrology 49, 1113–1124. Shaw, J., 2005. Geomorphic evidence of postglacial terrestrial environments on Atlantic Canadian continental shelves. Ge´ographie physique et Quaternaire 59, 141–154. Shaw, J., Forbes, D.L., 1995. The post-glacial relative sea-level lowstand in Newfoundland. Canadian Journal of Earth Sciences 32, 1308–1330. Shaw, J., Gareau, P., Courtney, R.C., 2002a. Paleogeography of Atlantic Canada 13–0 kyr. Quaternary Science Reviews 21 (16–17), 1861–1878.
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Shaw, J., Parrott, D.R., Patton, E., Atkinson, A., Beaver, D., Robertson, A., Girouard, P., 2005. Report on Cruise 2003-015: CCGS Matthew surveys in the Bras d’Or Lakes, Nova Scotia, 10–24 May 2003. Geological Survey of Canada Open File 5025. Shaw, J., Piper, D.J.W., Taylor, R.B., 2002b. Marine Geology of the Bras d’Or Lakes, Nova Scotia. Proceedings of the Nova Scotia Institute of Science 42, 127–147. Shaw, J., Taylor, R.B., Forbes, D.L., 1993. Impact of the Holocene Transgression on the Atlantic Coastline of Nova Scotia. Ge´ographie physique et Quaternaire 47, 221–238. Shaw, J., Taylor, R.B., Patton, E., Potter, D.P., Parkes, G.S., Hayward, S., 2006. The Bras d’Or Lakes, Nova Scotia: seafloor topography, backscatter strength, coastline classification, and sensitivity of coasts to sea-level rise. Geological Survey of Canada Open File Report 5397, compact disk. Stea, R.R., Conley, H., Brown, Y., 1992. Surficial Geology of the Province of Nova Scotia. Nova Scotia Department of Natural Resources. Map 92-3, scale 1:500,000.
Taylor, R.B., Shaw, J., 2002. Coastal character and model for coastal barrier evolution in the Bras d’Or Lakes, Nova Scotia. In: Petrie, B. (Ed.), The Bras d’Or Lakes. Proceedings of the Nova Scotia Institute of Science, vol. 42, pp. 149–181. Taylor, R.B., Frobel, D., 1998. Aerial video surveys: the Bras d’Or Lakes Shoreline, Nova Scotia. Geological Survey of Canada, Open File 3656, 58 pp. Taylor, R.B., Frobel, D., 2005. Cruise report 2004-302: coastal investigations of the Bras d’Or Lakes, Nova Scotia (Sensitivity to a rising sea level). Geological Survey of Canada, Open File 5007, 30 pp. Wang, Y., Piper, D.J.W., 1982. Dynamic Geomorphology of the Drumlin Coast of Southeast Cape Breton Island. Maritime Sediments and Atlantic Geology 18, 3–27. Woodman, J.E., 1899. Shore development in the Bras d’Or Lakes. The American Geologist 24, 329–342.
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Submerged early Holocene coastal and terrestrial landforms on the inner shelves of Atlantic Canada John Shaw a, *, Gordon B. Fader b, Robert B. Taylor a a b
Natural Resources Canada, Geological Survey of Canada, Atlantic, Bedford Institute of Oceanography, 1 Challenger Drive, Dartmouth, Nova Scotia, Canada B2Y 4A2 Atlantic Marine Geological Consulting Ltd., Halifax, Nova Scotia, Canada B3L 2Z2
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 22 August 2008
Coastal and terrestrial landforms that formed with lowered relative sea levels during the early postglacial period in Atlantic Canada were submerged during the Holocene transgression. However, these landforms are seldom seen on sea floor imagery. Factors that contribute to their destruction include the brevity of sea level lowstands and high wave energy on shallow modern shelves. We identify one situation within which preservation has been relatively good: large coastal lakes that existed for many thousands of years before being connected to the ocean by rising sea level in the mid-Holocene. We describe Bedford Basin, near Halifax, Nova Scotia, and deal more exhaustively with the Bras d’Or Lakes, an inland sea in Cape Breton, Nova Scotia. The preservation of shore platforms, barrier beaches and spits, and fluvial systems, was due to the rapid onset of the transgression and the relatively low wave energy in the subsequent marine phases. The well-preserved early Holocene coastlines are highly favourable targets in the search for evidence of human occupation in the early- to mid-Holocene. Ó 2008 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Although large areas of the continental shelves around Atlantic Canada (Fig. 1) were emergent at various times in the postglacial period (Shaw et al., 2002a), geophysical surveys rarely yield evidence of submerged shorelines. This is arguably due to the effects of high wave energy during the Holocene transgression. On the high wave energy Atlantic coasts of Nova Scotia, for example, where relative sea level (rsl) is rising at about 36 cm/century (Shaw et al., 1993), extra-tropical storms and hurricanes trigger rapid migration of coastal systems and erosion of coastal bluffs (e.g. Boyd et al., 1987; Carter et al., 1987, 1990, 1992, 1995; Forbes et al., 1990; Orford et al., 1991). The ongoing transgression destroys coastal landforms, leaving behind only truncated and buried estuarine deposits below thin surface lags (Forbes et al., 1991), or drumlin scars (Wang and Piper, 1982). Shaw (2005) described evidence of fluvial, deltaic and coastal systems on the shelves of Atlantic Canada. He argued that although large areas of the outer continental shelves were emergent for many thousands of years (see maps in Shaw et al., 2002a), lowstands were of insufficient duration to develop large coastal landforms that might survive transgression. Nevertheless, coastal landforms had been preserved in special circumstances (Forbes et al., 1995), and remnant early Holocene
* Corresponding author. Tel.: þ1 902 4266204; fax: þ1 902 4264104. E-mail address: [email protected] (J. Shaw). 1040-6182/$ – see front matter Ó 2008 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2008.07.017
fluvial systems have been found in a few areas, notably between New Brunswick and Prince Edward Island (Shaw, 2005). Submerged early Holocene deltas are found in southern Newfoundland, where they were created and preserved by unique circumstances, namely rapid relative sea level fall to a brief lowstand, abundant sediment supply in fiord-head glaciomarine deltas, and low wave energy (Shaw and Forbes, 1995). The settings in which well-preserved early Holocene deltas, fluvial channels, and coastal landforms are most likely to be found are large marine embayments that were formerly coastal freshwater lakes in the early Holocene (Shaw, 2005). In this paper we describe coastal and other landforms that have been preserved in two such settings: Bedford Basin, near Halifax, Nova Scotia, and the Bras d’Or Lakes, an inland sea in Cape Breton, Nova Scotia (Fig. 1). We examine the factors that led to the creation and preservation of these early Holocene shorelines and coastal landscapes, and note the implications for archaeology. 2. Study areas In this paper we focus on two areas: Bedford Basin and the Bras d’Or Lakes, both in Nova Scotia, Canada (Fig. 1). Bedford Basin (Fig. 2) is a large, semi-enclosed bay measuring approximately 7 km by 3 km, with a maximum depth of 71 m in the centre of the bay. Tidal range is up to a maximum of 2 m, and wind waves in the basin seldom exceed 1 m in height due to the restricted fetch. The basin is connected to Halifax Harbour and the Atlantic Ocean by The
J. Shaw et al. / Quaternary International 206 (2009) 24–34
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Fig. 1. Map showing location of Bedford Basin (A) and the Bras d’Or Lakes (B).
Narrows, a constricted 20-m deep channel, but was a freshwater lake until the mid-Holocene (Miller et al., 1982). The marine geology of the basin was described by Fader et al. (1991), Fader and Buckley (1995), Miller and Fader (1995) and in a forthcoming Geological Survey of Canada (GSC) Bulletin (Fader and Miller, in press). The Bras d’Or Lakes is the unofficial name for the inland sea on Cape Breton Island, Nova Scotia (Fig. 5) that comprises two large bodies of water: Bras d’Or Lake, in the south, and Great Bras d’Or, in the north. They are connected by Barra Strait. The principal connection with the ocean, Great Bras d’Or Channel, provides 95% of the exchange of marine water and has a sill depth of about 8 m (Canadian Hydrographic Service, 1996). The lakes are saline, but salinity decreases to about 21 parts per thousand in places (Petrie and Bugden, 2002). The circulation is predominantly inwards at the sea floor, and seawards at the surface. Tidal range is attenuated to only a few centimeters. In the last several years, the lakes have been surveyed using multibeam bathymetry systems, with the goal of producing 1:50,000 scale maps of bathymetry, backscatter and surficial geology. In addition to a suite of glacial landforms, the new imagery revealed extensive evidence of submerged shorelines, wave-cut platforms, river valleys and deltas (Shaw et al., 2006). The preserved features are mostly 25 m below modern sea level, far below the modern sill at 8 m. 3. Methods Bedford Basin (Figs. 2–4) was surveyed by the Canadian Hydrographic Service using multibeam bathymetry systems. A 5-m gridded data set supplied by the Department of Fisheries and Oceans Canada was viewed and analyzed in the Global Mapper Geographic Information System (GIS). Bedford Basin has been surveyed with various acoustic systems and a comprehensive suite of bottom samples has been obtained (Fader and Miller, in press). Multibeam bathymetric data were collected in the Bras d’Or Lakes (Fig. 1) in 2002, 2003 and 2004 using Canadian Hydrographic
Service launches with a Simrad EM-3002 system, and the survey vessel CCGS Creed, deploying a Simrad EM-1002 system. Denys Basin (Fig. 5) was surveyed in 2004 by a hydrographic survey launch with a SEA interferometric sidescan sonar system. Data were cleaned using Caris HIPS (Hydrographic Information Processing System). Processed depths were imported into the Geological Survey of Canada (GSC) version of the US Army Corps of Engineers GRASS GIS system. They were gridded, given sunillumination, and analyzed. Multibeam and interferometric data were processed to extract backscatter, a proxy for sediment type. Ground-truthing information came from a geophysical survey by CCGS Dawson in 1985 (Lynch, 1995), a geophysical survey by the first author in 1995 using CCGS Hart, gravity coring and geophysical surveys in the fall of 2002 (Parrott, in press), and a geophysical and sampling survey in 2003 (Shaw et al., 2005). One core was analyzed for foraminifera, and several were analyzed for macrofossil content by Paleotec Services of Ottawa, Ontario. Radiocarbon dates were calibrated with Calib 5.0.1 (Table 1). GSC Open File 5397 (Shaw et al., 2006) contains an ArcGIS project with images of the multibeam bathymetry and backscatter for the Bras d’Or Lakes, and an image with water level at 25 m. It also contains a detailed report on methods and results, a detailed description of coastal types, and an analysis of future water level trends and impacts. An understanding of modern coastal morphology and dynamics was based in part on aerial video surveys (Taylor and Frobel, 1998) and ground surveys (Taylor and Frobel, 2005). 4. Environmental change in Bedford Basin 4.1. Water levels In the early Holocene, relative sea level was low in the Halifax area (see Shaw et al., 1993; Edgecombe et al., 1999) and the basin existed as a lake. Micropaleontological data show that because of rising sea levels the lake became marine (Miller et al., 1982; Fader and Buckley, 1995, 1997). The transition to marine sedimentation is
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J. Shaw et al. / Quaternary International 206 (2009) 24–34
Fig. 2. Multibeam image of Bedford Basin. The inset map shows the extent of the freshwater lake during the early Holocene. To the northwest of the white dashed line the early- to mid-Holocene shoreline is buried by mud. The white arrow indicates the location of a bedrock ridge that was exposed during the relative sea level lowstand. The large white box delineates the extent of Fig. 3 and the small white box that of Fig. 4. Data courtesy of the Canadian Hydrographic Service.
dated at 5830 230 14C yr BP (Miller et al., 1982). Based on the sill depth at The Narrows, it was estimated that relative sea level had risen w20 m since that time. Part of the Halifax relative sea level curve is shown in Fig. 7. 4.2. Changes in geography The margins of Bedford Basin (Fig. 2) are somewhat rugged, with sea floor exposures of bedrock and thin till (Fader and Miller, in press). The centre of the basin is deep and muddy, with low-relief. A gently sloping terrace eroded into till, with an inflexion at 21 m below Chart Datum (CD) can be traced around the basin several hundred metres offshore; it can be traced around several former islands on the west side of the basin (Fig. 3). The upper limit of the terrace is a 0.5 to 1.0 m high boulder berm, while its lower limit is a line of boulders with little or no relief (Fig. 4). We believe that the terrace formed as a result of wave-erosion, and is indicative of an early Holocene lake level at 21 m CD. The extent of the former lake is shown on the inset in Fig. 2. Chart datum is approximately the level of modern low tides, so the former lake level was w22 m below modern mean sea level. The upper berm was probably created by a combination of ice-push and thermal expansion (see Gilbert, 1990) in a lake setting over an
extended period. Several hypotheses were considered for the lower line of boulders, namely: (1) it is a boulder barricade (Rosen, 1979) that rapidly formed (Gilbert, 1990) when the lakes became tidal; and (2) it formed when lake level dropped slightly as the sill was being breached by rising sea levels. The most likely explanation is that this low-relief boulder ridge is the exposed edge of a bouldermantled till surface that is buried by postglacial mud at a level deeper than the terrace (Fig. 4). The submerged shoreline disappears below a cover of lateHolocene mud in the northwest of the basin, in Bedford Bay near the channel of the Sackville River (i.e. beyond the dashed line on Fig. 2). A series of bedrock ridges extending across the basin in this region (arrow on Fig. 2; see also inset map) impounded a second lake during the early Holocene. It stood a few metres higher than the main lake and drained into the latter via a narrow rocky channel with rapids or waterfalls. 5. Environmental change in the Bras d’Or Lakes 5.1. Water levels Analyzing data from the 1985 survey (Lynch, 1995), HillaireMarcel (1987) reported that during the postglacial period the lake
J. Shaw et al. / Quaternary International 206 (2009) 24–34
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Fig. 3. Submerged shorelines on the west side of Bedford Basin. Data courtesy of the Canadian Hydrographic Service.
had experienced a marine phase, a freshwater phase, and a final marine phase (see also Lortie, 1987). The onset of the final freshwater phase was undated. De Vernal and Jette´ (1987) concluded that the onset of freshwater conditions was ca. 4500 to 5000 14c yr BP. Given that the sill at the mouth of the lakes was at a depth of 8 m, it was estimated that the rate of relative sea level increase since the onset of marine conditions had been 16–20 cm/ century. Shaw et al. (2002b) tried to reconcile Grant’s (1994) conclusions that late glacial sea levels were low in the area with Hillaire-Marcel’s (1987) evidence that they were high at the beginning of the Holocene. They argued that the early Holocene freshwater lake in
the area stood 25 m below modern sea level. Because the modern sill in Great Bras d’Or Channel is only 8 m deep they proposed that the freshwater lake drained to the ocean via a minor Channel, Little Bras d’Or Channel (Fig. 5). Although the modern sill connecting the Bras d’Or Lakes (Fig. 5) with the Atlantic Ocean, in Great Bras d’Or Channel, is only 8 m deep, sub-bottom profiling in the channel reveals that a former bedrock sill at 25 m is buried beneath a prograded wedge of sand (Fig. 6, top) interpreted as an elongated flood-tidal delta (Shaw et al., 2006). The source of sediment in this delta is sand eroded from high, unconsolidated bluffs immediately outside the entrance to the lakes (Taylor and Frobel, 2005) and transported into the channel by tidal currents that have a strong
Fig. 4. Sidescan sonogram showing the submerged shoreline, comprising two lines of boulders and intervening terrace. Light tones are less reflective sea floor (mud) while dark tones are highly reflective (boulders, gravel).
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Fig. 5. The Bras d’Or Lakes, with locations of figures, and locations of the two seismic profiles shown in Fig. 6. Inset map shows the extent of freshwater lakes in the early Holocene.
landward asymmetry at the seabed (Petrie and Bugden, 2002). This evidence shows that the former sill to the lakes is 25 m below modern sea level, and not 8 m as might be supposed. Coring in St. Patrick’s Channel (Fig. 5) reveals marine mud up to 3 m thick containing scattered shell fragments, marine bivalves (Cerastoderma pinnulata), and estuarine foraminifera whose abundance decreases downcore. The marine mud was deposited under conditions of increasing marine influence through time. Underlying the mud are layers of gray clay interbedded with peat and gyttja containing twigs and tree leaves, with seeds of aquatic plants such as Typha sp., Sparganium americanum and the shrub Alnus incana. A few foraminifera were noted in some parts of the organic-rich units, but not in others. These organic-rich units accumulated in fresh and brackish conditions, probably in a series of marshes and ponds on the flat floor of what is now St. Patrick’s Channel. The generalized relative sea level curve for the lakes (Fig. 7) is primarily based on nine samples from the organic layers (Shaw et al., 2006), together with two dates on shells in the overlying
marine mud (Table 1). Four dates (12–15) published by Miller and Livingstone (1993) have been added. These dates come from a small cove mid-way along Great Bras d’Or Channel, connected to the channel via a sill that is 1.5 m below the high tide level. The dates showed that sea level was 1.5 m below present level at 950 BP. 5.2. Geographic changes The inset on Fig. 5 shows the geography of the Bras d’Or Lakes region during the early- to mid-Holocene, based on multibeam bathymetry data with a 25 m lowering of water level applied. In the north, St. Patrick’s Channel was subaerially exposed, and contained a river that drained into the lake at the east end of the channel, where it formed a large delta (Fig. 6, bottom). At the west end of the channel, modern basins were occupied by lakes at higher levels. In the southern lakes, modern East Bay contains several basins that, in the early- to mid-Holocene, contained lakes at elevations slightly above elevations of the main lake (24 m), and were
Table 1 Radiocarbon dates on samples from St. Patrick’s Channel, Bras d’Or Lakes. #
Core and depth
Material dated
Depth (m)
Lab #
13
Age
Error
Calibrated age BP (two sigma range)
1 2 3 4 5 6
2002066-090-070 2002066-090-140 2002066-090-152 2002066-090-160 2002066-094-123 2002066-094-155
Shell Twig Wood Twig Peat Wood
14.3 15.0 15.1 15.2 13.2 13.6
185984 197660 185985 197661 185986 185987
C/12C ratio
0.3 25.3 28.1 26 27.7 27.3
2120 4440 4510 4480 4110 4090
40 40 40 40 40 40
7 8 9 10 11
2002066-096-074 2002066-096-122 2002066-098-064 2003015-077-160 2003015-077-170
Shell Peat Peat Wood Twig
13.9 14.4 11.5 21.8 21.9
185988 185989 185990 185991 197662
0.5 28.1 27.7 28.2 28.2
2330 4220 3900 5250 5370
60 40 40 40 40
1411–1659 4876–5083; 5039–5310 4975–5017; 4453–4461; 4442–4483; 4718–4725; 1621–1931 4624–4763; 4160–4168; 5922–6121; 6004–6083;
5103–5138; 5162–5282 5031–5297 4520–4729; 4735–4741; 4750–4821 4512–4711 4753–4815 4787–4857 4179–4199; 4229–4428 6146–6178 6099–6160; 6170–6280
Numbers in column one refer to locations illustrated in Shaw et al. (2006). Column two shows the core number and sample depth down core. Sample depths are reduced to Chart Datum, which is lowest normal tide. Tidal range is about 0.1 m. Calibrated ages were obtained using Calib 5.0.1. The woody samples were calibrated using intcal04.14c and the shell samples using marine04.14c with local Delta R ¼ 140 (McNeely et al., 2006).
J. Shaw et al. / Quaternary International 206 (2009) 24–34
Fig. 6. Acoustic profiles in the Bras d’Or Lakes. Top: Seistec high-resolution sub-bottom profile across the flood-tidal delta inside the entrance to the lakes. (A) Flood-tidal delta; (B) bedrock; (C) masking by shallow gas. Bottom: Huntec high-resolution deeptowed sub-bottom profile across the delta at the mouth of St. Patrick’s Channel. (D) Postglacial marine mud; (E) glaciomarine and glaciolacustrine/lacustrine mud; the upper part of this sediment package grades into the delta foresets (F) at the west side of the profile; (G) bedrock or possibly till.
connected to it by rivers. Denys Basin (Fig. 5) was emergent, and was drained by a meandering river that formed a large delta where it reached the lake. Large fields of drumlins in the modern West Bay formed a complex network of islands connected by barrier beaches, with narrow, winding channels that connected with the main part of the ancient lakes. 5.3. Submerged coastal landforms The Bras d’Or Lakes existed for many thousands of years as freshwater lakes, and developed coastal landforms easily identifiable on multibeam sonar imagery and similar in size, morphology and lithology to their modern counterparts in the lakes (see Woodman, 1899; Taylor and Shaw, 2002; Taylor and Frobel, 2005). The greater abundance of coastal landforms in the southern lake (Bras d’Or lake) is explained by the prevalence of drumlin fields
Fig. 7. Simplified trend of relative sea level in the Bras d’Or Lakes during the middle- to late-Holocene epoch. The curve is poorly constrained after ca. 4000 BP. Samples 12–15 are from Miller and Livingstone (1993), who argued that sea level was 1.5 m below present at 950 BP. The curve for Halifax (dotted line) is based on Edgecombe et al. (1999).
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there, and their absence in the northern lake (Great Bras d’Or). These are the underwater extension of drumlin fields mapped on land by Grant (1988, 1994) and Stea et al. (1992), which acted as sediment supplies for coastal landforms (Taylor and Shaw, 2002; Shaw et al., 2006). East Bay (Fig. 8) contains tombolos, spits, and barrier beaches, mostly graded to the level of 24 m, slightly shallower than the lakes north of Barra Strait (25 m). They have high backscatter, and are composed of sub-rounded pebbly gravel. They appear on seismic profiles superimposed on earlier deposits. In some instances barrier beaches are located at shallower depths, indicating that with rising sea levels they migrated until they were trapped by topography (see Fig. 8D). In West Bay, fields of drumlins were modified by littoral processes to produce coastal landforms that were subsequently submerged. They include barrier beaches linking drumlin headlands (Fig. 9). They have high backscatter, and formed with a 24 m water level, again slightly shallower than north of Barra Strait. West Bay also contains extensive submerged shore platforms at 24 m. These are incised into drumlins, and formed under the influence of waves generated by a relatively large (12 km) fetch to the southwest. The platforms are poorly developed on the northeast sides of drumlins. Except for possibly one area, ridges at the upper limits of the platforms (as in Bedford Basin) are absent. A distinctive aspect of West Bay is the presence of ‘tails’ attached to submerged drumlin shoals (Fig. 9). During the lake phase some drumlins had trailing spits attached. As sea level rose, the spits migrated to the rear of the shoals, coalesced, and formed attached ‘tails’ that were eventually trapped by topography and drowned. The tails are generally elongate, but in places the sediment trapped in the lee of a drumlin forms a spillover wedge. In a conceptual model for barrier inception, growth and destruction in a rising sea level setting in the Bras d’Or Lakes (Taylor and Shaw, 2002; Shaw et al., 2006), these ‘tails’ represent the stranding of sediment in the lee of a drumlin that has become completely eroded and submerged. Modern examples are seen in Fig. 59 of Shaw et al. (2006). 5.4. Submerged fluvial systems and deltas Well-preserved submerged river systems are found in the lakes. In St. Patrick’s Channel an incised river valley is bounded by bluffs and has a braided channel floor (Fig. 10). The former river formed a delta a short distance to the southeast. The thickness of the deltaic wedge (Fig. 6) and its interfingering with strongly stratified sediments interpreted as both glaciomarine and glaciolacustrine in origin, suggests that it was initially sourced by glacial meltwater, when sea level had dropped below the 25 m sill level. Farther west the channel is overlain by mud, but can still be glimpsed on multibeam sonar imagery as a series of looped meanders. Yet farther west the channel is imaged below the 3-m thick mud on sub-bottom profiles. The low-gradient valley floor was poorly drained, and hosted marshes and ponds in which organic materials accumulated – i.e. the materials sampled and dated for the sea level curve. A second submerged fluvial channel system is seen in Denys Basin (Fig. 11). Knudsen 28 kHz sounder data show that the channels lie below a thin blanket of late-Holocene mud. This river reached the former lake and formed a delta, which today is an extensive flat area at a depth of 24 to 25 m. The surface has a mud veneer, through which meandering channels are vaguely discernable. The size of this delta suggests that it also was partly formed by meltwater discharged by glacial ice located farther west. 5.5. Submerged bioherms Surveys in Denys Basin using the interferometric sidescan system revealed numerous acoustically reflective, circular to oval
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Fig. 8. Multibeam bathymetry image showing submerged coastal landforms in East Bay. These include a tombola (A), spits (B), barrier beaches (C), and a relatively shallow barrier beach (D) that was stranded during the process of migration. The structures at E are probably barriers enclosing lagoons, and are commensurate with the modern analogs in the region (see Taylor and Shaw, 2002). All these landforms have high backscatter, and bottom photographs and samples show that they are composed of gravel or sandy gravel.
Fig. 9. Multibeam image from West Bay, showing the submerged erosional terraces (A) together with a barrier beach (B). When the lake stood at 24 m, the narrow channel at C connected the lake in the southwest (modern West Bay) with the main lake. The trailing ‘tails’ attached to submerged drumlins are shown at D.
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Fig. 10. Multibeam image of a submerged river system in St. Patrick’s Channel. The channel terminates in a delta at 25 m just off the image at right. It appears to show gravel bars (A) bounded by bluffs (B). Towards the southwest the channels disappear beneath a cover of mud, but can be vaguely discerned on the multibeam imagery as a series of looped meanders, and an oxbow (C).
marks on the sea floor (Fig. 12, top). Bottom video and sampling (G. Bugden, personal communication, 2006) reveals that these features are circular to oval bioherms of the American oyster, Crassostrea virginica. These contrast with the linear bioherms reported in New England by Carbotte et al. (2004). Coring reveals the bioherms as vertical structures composed of oyster shells and interstitial mud, surrounded by mud. Sub-bottom profiles with a Knudsen dualfrequency sounder (Fig. 12, bottom) show that the bioherms are commonly 3 m high, and are ‘rooted’ on a hard substrate below the acoustically transparent Holocene mud. 6. Discussion 6.1. Erosional terraces There are significant differences between the submerged erosional terraces in the two study areas. The Bras d’Or Lakes submerged terraces are relatively wide (80 m) and formed in an environment with large fetch (up to 12 km) and abundant sediment supply in the form of numerous drumlins. As a result, coastal barriers and spits were as common in the lake as they are in the modern inland sea. The abundance of sediment is reflected in the presence of the drumlin tails and spillover deposits that were trapped as individual drumlins were submerged. The Bedford Basin shoreline, by contrast, was developed in an area of limited fetch (2 km), and thin till, so coastal landforms did not develop. The boulder ridge at the upper edge of the Bedford Basin terraces may represent a back-terrace ridge that was abandoned during the rapid transgression.
6.2. Drowning and preservation of coastal landforms The drowning of early Holocene shorelines at the two study sites is the result of the instantaneous onset of rapid sea level rise, when the Holocene transgression breached the sills. When the sills at Bedford Basin and the Bras d’Or Lakes were breached (ca. 6650 and 6350 yr cal BP), the respective rates of sea level rise were about 67 and 79 cm/century, respectively. For comparison, the modern rate of sea level rise at Halifax, Nova Scotia, is about 36 cm/century (Shaw et al., 1993). Both water bodies were thus subject to an instantaneous and rapid increase in water level, accounting for the good preservation of coastal landforms. Although most coastal landforms in the Bras d’Or Lakes drowned rapidly, there is, nevertheless, evidence that some adjusted to sea level rise by migrating. Coastal features identifiable on the multibeam imagery are at depths from 7 to 25 m. The majority are in the range 15–20 m, with a nearly equal number in the range 20–25 m. Drowning of coastal systems thus occurred mainly before 4500 cal yr BP, and most drowning was in the period 5500 to 6000 cal yr BP. The rate of water level increase at 4500 BP was 57 cm/century, and from 5500 to 6000 BP it ranged from 69 to 75 cm/century. 6.3. Oyster bioherms The oyster colonies clearly formed after 6350 yr cal BP, when the lakes became connected with the ocean. The river channels in this part of Denys Basin extend to depths of 14 m, and would have become brackish ca. 4500 cal yr BP. The bases of the bioherms are on average 8 m below sea level, suggesting that they could have
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Fig. 11. Multibeam image showing the submerged fluvial system in Denys Basin, buried under several metres of postglacial mud. Fig. 12 is located just off this figure, in the extreme southwest.
Fig. 12. Top: backscatter in part of Denys Basin, derived from an interferometric survey system. The black circles and ovals are oyster bioherms – colonies up to 25 m in diameter. Bottom: sub-bottom reflection profile across the bioherms. The horizontal dashed lines are 2 m apart.
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been established after 3500 cal yr BP. We infer, therefore, that oysters could have migrated into this area after 4500 cal yr BP, and were possibly established by 3500 cal yr BP. 6.4. Archaeological implications Murphy (1998) noted that in the prehistoric culture sequence of the Maritime Provinces of Atlantic Canada (New Brunswick, Nova Scotia, and Prince Edward Island) there are no professionally excavated sites dating to the Early and Middle Archaic periods (i.e. 10,000–5000 BP). It is likely that occupational evidence was formerly located along shorelines that were submerged during the Holocene transgression. As noted in Section 1, the high wave energy of the transgression destroyed former coastlines. However, the two examples described here show that ancient coastlines have been well-preserved in the special circumstances of large coastal lakes that were stable for many thousands of years, until they connected with the ocean in the mid-Holocene. Based on the concepts elucidated by Pedersen et al. (1997) it should be possible to identify locations with a high potential for former human occupation in both the Bras d’Or lakes and Bedford Basin. 7. Conclusions (1) In the early Holocene, Bras d’Or Lakes and Bedford Basin, Nova Scotia were freshwater lakes. They connected with the ocean at ca. 6350 and 6650 cal yr BP, respectively, due to rising sea levels. (2) Shoreline systems that developed in these large lakes over many thousands of years were preserved due to instantaneous onset of rapidly rising water levels. This contrasts with the more exposed continental shelves in the region, where wellpreserved coastal systems are rare. (3) The submerged shorelines in Bedford Basin consist of shore platforms at 21 m; more complex shores at around 25 m and above were developed in the Bras d’Or Lakes because of the presence of drumlin fields that acted as sediment sources. Some coastal systems were able to migrate landward for a while before drowning. (4) We believe that these submerged landscapes and features provide high-priority targets for the search for early human occupation of the region.
Acknowledgements We thank David Frobel for assistance with coastal videos and coastal fieldwork. Russell Parrot and David Piper provided helpful internal reviews. Stanley Johnson of the Department of Fisheries and Oceans Canada (DFO) provided data files for Bedford Basin and Gary Bugden (DFO) gave information on the bioherms in Denys Basin. This research was part of Natural Resources Canada Project X29 in the Geoscience for Ocean management Program and is Earth Sciences Sector Contribution 20070549. References Boyd, R., Bowen, A.J., Hall, R.K., 1987. An evolutionary model for transgressive sedimentation on the Eastern Shore of Nova Scotia. In: Fitzgerald, D.M., Rosen, P.S. (Eds.), Glaciated Coasts. Academic Press, San Diego, pp. 87–114. Canadian Hydrographic Service, 1996. Chart 4277: Great Bras d’Or, St. Andrews Channel, and/et St. Anns Bay, scale 1:40,000. Carbotte, S.M., Bell, R.E., Ryan, W.B.F., McHugh, C., Slagle, A., Nitsche, F., Rubenstone, J., 2004. Environmental change and oyster colonization within the Hudson River estuary linked to Holocene climate. Geo-Marine Letters 24, 212–224. Carter, R.W.G., Shaw, J., Jennings, S.C.,1995. Morphodynamic evolution, self-organisation and instability of coarse-clastic barriers on paraglacial coasts. Marine Geology 126, 63–85.
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